Diffuse large B-cell lymphoma (DLBCL) is the most common type of non-Hodgkin’s lymphoma, for which cyclophosphamide, doxorubicin, vincristine, and prednisone with rituximab(R–CHOP) is one of the standard regimens. Given that R–CHOP is highly emetogenic, chemotherapy-induced nausea and vomiting (CINV) prevention is clinically important. However, there is a paucity of studies focusing on these patients. This study aimed to ascertain the effectiveness of an oral fixed-dose combination of netupitant and palonosetron (NEPA) in preventing CINV in patients with DLBCL undergoing first-line R-CHOP chemotherapy. Seventy patients were enrolled in this single-center prospective non-comparative study conducted between November 2020 and May 2023 in South Korea. NEPA was administered 1 h prior to chemotherapy initiation on day 1. The primary endpoint of the study was the complete response rate (no emesis, and no rescue medication) during the acute, delayed, and overall phases, which were assessed over a period of 120 h post-chemotherapy. The complete response rates for NEPA were 90.0% [95% CI 80.5, 95.9] for the acute phase, 85.7% [95% CI 75.3, 92.9] for the delayed phase, and 84.3% [95% CI 73.6, 91.9] for the overall phase, with no-emesis rates (acute: 97.1% [95% CI 97.1, 99.7], delayed: 95.7% [95% CI 88.0, 99.1], overall: 92.9% [95% CI 84.1, 97.6]). NEPA was well tolerated with no severe treatment-emergent adverse events. NEPA exhibited substantial efficacy in mitigating CINV in DLBCL patients undergoing R–CHOP chemotherapy, demonstrating high CR and no-emesis rates, and favorable safety profiles.